Synthetic magnesium silicate compositions and process for the production thereof

ABSTRACT

A synthetic magnesium silicate composition of the formula: 
 
{[Si 8 (Mg a Li b )O 20 (OH) 4-c F c ] d−   d/n M n+ } w {Na 2 SO 4 } x {Na 2 CO 3 } y {H 2 O} z 
where a is 4.5 to &lt;6.0, b is from &gt;0 to 1.5, c is from 0 to 4, d=12−2a−b, M is a cation, n is the valency of cation M, w is from 32.8 to 94.7 wt. %, x is from 0.3 to 32.0 wt. %, y is from 0 to 9.0 wt. %, and z is from 0 to 50 wt. %, is useful in paraffin wax dispersions as it does not discolour or yellow such dispersions

BACKGROUND

This invention is concerned with synthetic magnesium silicatecompositions and a process for the production thereof.

With the name “hectorite” has been ascribed to a natural trioctahedralsmectite found at Hector, Calif., USA. This clay is an hydrous magnesiumsilicate having the ideal composition Si₈ Mg₆O₂₀(OH)₄ modified by havinga portion of the Mg⁺² and OH⁻ ions replaced by Li+ and F− ions.

The synthesis of hydrous magnesium silicates similar to naturalhectorite has been described by Granquist and Pollack in “Clays and Clayminerals” Vol. 8 (Proceedings of the 8^(th) National Conference on Claysand Clay Minerals) pages 150-169. In the process described by Granquist,gels of magnesium hydroxide and of silica are produced separately, arewashed, are combined and are redispersed in water to form a suspension.Lithium hydroxide or lithium fluoride and sodium hydroxide are added tothe suspension that is then treated hydrothermally by refluxing it withstirring until a product having a crystal structure similar to that ofhectorite is formed.

While Granquist's product has the crystal structure similar to naturalhectorite it does not have good rheological properties. Measuring theBingham Yield Value of an aqueous dispersion of the substance provides astandard yardstick of theological properties of a substance. The termBingham Yield Value (also known as Bingham Yield Stress, these termsbeing alternatives for the same property) is referred to in standardworks on rheology for example in “Rheology Theory and Applications” F REirich (Acad. Press) Vol. 1 (1956) page 658 and “Colloidal Dispersions”L K Fisher (N.Y. Bureau of Standards) 2^(nd) Edition 1953, pages 150-170and “The Chemistry and Physics of Clays and other Ceramic Materials”3^(rd) Edition, page 463, A B Searle and R W Grimshaw.

The Bingham Yield Value may be determined by first obtaining a flowcurve relating the shear stress to the rate of shear and thenextrapolating the straight line section of the curve to the shear stressaxis, the intercept being the Bingham Yield Value. It can convenientlybe determined on any viscometer capable of measuring a range of shearrates and shear stresses.

The product of Granquist, when in the form of a dispersion obtainedusing 2 g silicate and 100 ml tap water, gives a Bingham Yield Value ofonly about 15 dynes per cm 2. This is a very low value, inferior to thatgiven by natural hectorite. It's also gives a low static gel strength.

Processes for the production of synthetic hydrous magnesium silicatecompositions having a crystal structure similar to natural hectorite buthaving better rheological properties than natural hectorite have beendescribed in GB-A-1054111, GB-A-to1213122 and GB-A-1432770.

The process described in the GB-A-1054111 involves forming a slurry byco-precipitation by slowly combining with heating and agitation in anaqueous medium a constituent providing the magnesium ions withconstituents providing the silicon (as silicates), hydroxyl and sodiumions and treating the precipitate hydrothermally to crystallise thesynthetic mineral-like clay, washing and dewatering the resultingcrystallised product, and drying the product at a temperature up to 450°C. The concentration of the slurry is desirably such that theconcentration of the product formed is from 1% to 8% by weight,preferably 4% by weight. The hydrous magnesium silicate containsfluorine and lithium. The clay-like minerals provided have thestructural formula:(Si₈Mg_(6-x)Li_(x).O₂₀.(OH)_(4-y)F_(y))^(x(−)).x/nMn^(n(+))in which x is between 0 and 6, y is from 1 up to but excluding 4, and Mis a cation. Li⁺ may be replaced by Na⁺.

The process described in GB-A-1213122 involves precipitating a magnesiumsilicate by combining an aqueous solution of a water soluble magnesiumsalt with an aqueous alkaline solution of one or more sodium compoundsin the presence of dissolved silicon compound and hydrothermallytreating the precipitate under pressure to crystallise the syntheticmineral-like clay, separating the resultant solid and liquid phases,washing the resulting crystallised product, and drying the product. Theconcentration of the precipitate is preferably not more than 5% byweight. The hydrous magnesium silicate product contains no fluorine,optionally contains lithium and has the general formula:[Si₈Mg_(a)Li_(b)H_(4+c)O₂₄]^((12−2a−b−c)—).M^((12−2a−b−c)+)where (i) M is a sodium, a lithium or an equivalent of an organiccation, and (ii) the value of a, b, and c is such that either a<6, b>0,c>0, b+c<2, and (a+b+c−6)<2; or a<6, b=0, c, 2 and (a+c−6)<2.

The process described in GB-A-1432770 involves the synthesis of anhydrous magnesium silicate having a crystal structure similar to that ofhectorite and having the general formula:[Si₈(Mg_(z)Li_(b)H_(c))O₂₀(OH)_(4-y)F_(y)]^(z−).zM⁺wherein a is 4.95 to 5.7, b is from 0 to 1.05, c is from 0 to <2, a+b+cis from >4 to <8, y is from 0 to <4, z=12−2a−b−c, and M is Na+ or Li⁺.The process comprises the sequential steps of forming an aqueoussuspension of magnesium carbonate, forming a silica precipitate in theaqueous suspension magnesium carbonate, the proportions of magnesiumprovided by the magnesium carbonate and silica precipitated in thesuspension corresponding to that of the formula of the magnesiumsilicate, maintaining the resulting mixture of magnesium carbonate andsilica in the wet state and subjecting it to hydrothermal treatment byheating it in an aqueous medium and in the presence of the remainingconstituents of the magnesium silicate in proportions within the rangesspecified in the general formula and in the presence of excess dissolvedsodium or lithium compound over that required to form the cation of themagnesium silicate until crystal growth occurs and separating theresulting crystalline product. The crystalline material resulting fromthe hydrothermal treatment is then separated by filtration, washed, anddried at a temperature not exceeding 450° C. The process described inGB-A-1432770 is distinguished from the processes described inGB-A-1054111 and GB-A-1213122 in that, in those processes, the Mgcompound and the silica are co-precipitated.

The products of the processes described in the above prior art documentsare characterised by providing dispersions having Bingham Yield Valuessubstantially in excess of any known to be given by natural hectoritedispersions. However, it is known that small modifications of thecomposition of the prior art synthetic magnesium silicates or of theformulation of dispersions comprising such silicates can havesignificant deleterious effects upon these theological properties.

Some of the prior art products have found widespread use, by virtue oftheir excellent theological properties, in many applications, includingin paints; cosmetic products; shampoos; detergents; disinfectants;toothpastes; paper manufacture, for example as fillers, retention anddrainage aids, and in paper coatings; and drilling muds. The products ofthe above processes are commercially available as dry white powders,such as the products sold by Rockwood Additives Limited, England, underthe trade mark “LAPONITE”. When fully dispersed and hydrated in water,the resulting composition is colourless and transparent.

When the products of the above processes are dispersed in paraffin wax,however, there is a tendency for the wax dispersion to discolour over aperiod of time.

SUMMARY

It is an object of the present invention to provide synthetic magnesiumsilicate compositions that have similar colour and theologicalproperties as the prior art products but which, when dispersed inparaffin wax, exhibit reduced or eliminated discoloration of the waxdispersion.

It is a further object of the present invention to provide a process forthe preparation of such synthetic magnesium silicate compositions.

DETAILED DESCRIPTION

In accordance with the first aspect of present invention there isprovided a synthetic magnesium silicate composition of the formula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}_{w}\{ {{Na}_{2}{SO}_{4}} \}_{x}\{ {{Na}_{2}{CO}_{3}} \}_{y}\{ {H_{2}O} \}_{z}$wherein a is 4.5 to <6.0, b is from >0 to 1.5, c is from 0 to 4,d=12−2a−b, M is a cation, n is the valency of the cation M, w is from32.8 to 94.7% w/w, x is from 0.3 to 32.0% w/w, y is from 0 to 9.0% w/w,and z is from 0 to 50% w/w. Preferably a is between 5.30 and 5.68. MostPreferably a is between 5.42 and 5.55, and z is less than 2% w/w. M ispreferably selected from Na, K, Li, an organic cation, such as aquaternary ammonium anion eg {R₂(CH₃)₂}⁺, where R is C₁₄ to C₂₂,preferably C₁₈, alkyl, and mixtures thereof. More preferably M isselected from Na, Li and mixtures thereof. Most preferably M is Na.

As only small modifications of the composition of the prior artsynthetic magnesium silicates or of the formulation of dispersionscomprising such silicates can have significant deleterious effects uponthe rheological properties of compositions, it is surprising to findthat the synthetic magnesium silicate compositions of the presentinvention have retained colour and rheological properties as the priorart products and, importantly, demonstrate a significantly reduced, ifnot eliminated, discoloration effect on wax dispersions.

Whilst the physical make-up of the compositions of the present inventionis not fully understood, it is believed that of the compositions arecomplex, intimate, amorphous/crystalline blends of the relevant definedconstituents. Accordingly, the above formula should be determined at themicro level. In other words, the compositions of the present inventionare not simple dry blends of the defined constituents having aformulation corresponding to the above formula but determined at themacro level. Indeed, a dry blend of the relevant constituents having aformulation corresponding to the above formula when determined at themacro level, even when micronised to reduce the mixture particle size toless than 20 microns, simply does not demonstrate the advantages of thepresent invention, i.e. paraffin wax dispersions comprising such asimple blend will tend to discover or yellow.

In another aspect of the present invention, there is provided a processfor the preparation of a synthetic magnesium silicate composition of theformula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}_{w}\{ {{Na}_{2}{SO}_{4}} \}_{x}\{ {{Na}_{2}{CO}_{3}} \}_{y}\{ {H_{2}O} \}_{z}$wherein a is 4.5 to <6.0, b is from >0 to 1.5, c is from 0 to 4,d=12−2a−b, M is a cation, n is the valency of the cation M, w is from32.8 to 94.7% w/w, x is from 0.3 to 32.0% w/w, y is from 0 to 9.0% w/w,and z is from 0 to 50% w/w. Preferably a is between 5.30 and 5.68. Mostpreferably a is between 5.42 and 5.55, and z is less than 2 w/w. M ispreferably selected from Na, K, Li, an organic cation, such as aquaternary ammonium anion eg {R₂(CH₃)₂}⁺, where R is C₁₄ to C₂₂,preferably C₁₈, alkyl, and mixtures thereof. More preferably M isselected from Na, Li and mixtures thereof. Most preferably M is Na.

In both the composition and the process of the invention, it will beappreciated by a person skilled in the art that a+b may be less than orequal to 6.

In a first embodiment of the process of the invention, the processconsists essentially of the following sequential steps:

-   -   (i) forming an aqueous suspension of magnesium carbonate,    -   (ii) forming a silica precipitate in the aqueous suspension of        magnesium carbonate, the proportions of magnesium provided by        the magnesium carbonate and of silica precipitated in the        suspension corresponding to that of the formula of the magnesium        silicate,    -   (iii) whilst maintaining the resulting mixture of magnesium        carbonate and silica in the wet state, subjecting it to        hydrothermal treatment by heating it in an aqueous medium and in        the presence of the remaining constituents of the magnesium        silicate in proportions within the ranges specified in the        general formula thereof and in the presence of excess dissolved        sodium or lithium compound over that required to form the cation        thereof until crystal growth occurs,    -   (iv) separating the solid and liquid phases, and    -   (v) drying the resultant solid product at a temperature up to        450° C.

In a second embodiment of the process of the invention, the processconsists essentially of the following sequential steps:

-   (i) forming an aqueous slurry from    -   (a) a water-soluble magnesium salt,    -   (b) sodium silicate,    -   (c) sodium carbonate or sodium hydroxide and    -   (d) material delivering lithium and fluoride ions selected from        the group consisting of (A) lithium fluoride and (B) a lithium        compound in conjunction with hydrofluoric acid, fluosilicic        acid, sodium silicofluoride or sodium fluoride, such that in the        slurry the following atomic ratios are present        $\frac{Si}{F} = {0.5\quad{to}\quad 5.1}$        $\frac{Li}{Mg} = {0.1\quad{to}\quad 1.0}$        $\frac{Si}{{Mg} + {Li}} = {0.5\quad{to}\quad 1.5}$        $\frac{Na}{{2\quad{Mg}} + {F\text{-}{Li}}} = {1.0\quad{to}\quad 2.0}$    -   the aqueous slurry being formed by co-precipitation by slowly        combining the said magnesium salt and the said sodium silicate        and the said sodium carbonate or sodium hydroxide, with heating        and agitation, in an aqueous medium which contains the said        material or materials delivering the lithium and fluoride ions;-   (ii) taking the aqueous slurry so formed and, without washing free    from soluble salts, hydrothermally treating it for about 10 to 20    hours until crystal growth occurs,-   (iii) separating the solid and liquid phases, and-   (iv) drying the resultant solid product at a temperature up to    450° C. In a third embodiment of the process of the present    invention, the process consists essentially of include the following    sequential steps:    -   (i) precipitating a magnesium silicate having the value of “a”        desired in the said composition by combining an aqueous solution        of a water soluble magnesium salt with an aqueous alkaline        solution of one or more sodium compounds in the presence of        dissolved silicon-delivering material, the pH of the alkaline        solution being maintained at 8 to 12.5 throughout,    -   (ii) without first drying or washing this precipitate heating it        to a temperature of at least 170° C. and the pressure of at        least 6.9 bar (100 psi), the temperature being less than 370° C.        and such that a liquid phase is present, until crystal growth        occurs,    -   (iii) separating the resultant solid and liquid phases, and    -   (iv) drying the resultant solid product.

The compositions of the present invention retain similar theologicalproperties to the products formed in GB-A-1054111, GB-A-1213122 andGB-A-1432770. The compositions may be supplied as dry white powders oras moist solids or in dispersions. Accordingly, the compositions thepresent invention may be used in the same type of applications as theprior art products. For example, the compositions may be used in paints;cosmetic products; shampoos; detergents; disinfectants; toothpastes;paper manufacture, for example as fillers, retention and drainage aids,and in paper coatings; and drilling muds. Such compositions have theadvantage over the prior art products because the compositions do notyellow or discolour paraffin wax when dispersed therein.

EXAMPLES

The following examples illustrate aspects of the invention, but are notin any way intended to limit the scope the invention.

Example 1 Preparation of Compositions of the Present Invention

Various compositions of the present invention, as set out in Table 1,can be prepared by the following process. To determine the amounts of,for example, lithium carbonate, magnesium sulphate, sodium silicate, andsodium carbonate employed in the process reference should be made to theformula of the desired synthetic magnesium silicate. (Should a fluorinecontaining product be desired, reference should first be made to theformula of the desired synthetic magnesium silicate to determine thequantity of fluorine needed in the desired synthetic magnesium silicate,and the quantity of a suitable fluorine containing material required forthe process can then be determined.)

A measured quantity of lithium carbonate and water (sufficient todissolve the measured quantity of lithium carbonate) is placed in aflask fitted with a stirrer, a heating mantle and a refluxing condenser.In a separate vessel, a measured quantity of magnesium sulphate isdissolved in sufficient water such that the solution was almostsaturated and the solution added to the lithium carbonate solution. Themixture was brought to a temperature of at least 60° C. under refluxwhile stirring efficiently.

From a separate vessel a measured quantity of sodium carbonate solutionis added slowly to the reaction vessel containing the lithium carbonateand magnesium sulphate solution. The addition is made over a period ofup to one hour, while the reaction mixture is kept at 60° C. or greaterand stirred efficiently throughout.

From a separate vessel a measured quantity of sodium silicate solutionis added slowly to the reaction vessel containing the lithium carbonate,magnesium sulphate and sodium carbonate solution. The addition is madeover a period of up to one hour, while the reaction mixture is kept at60° C. or greater and stirred efficiently throughout.

The mixture is then boiled under reflux, with efficient stirring, forabout 2 hours. The mixture is then transferred to a pressure vessel andheated at 202° C. or greater for at least 6 hours. After that, it isfiltered under vacuum leaving a filter cake that is dried in trays atcirca 110° C. and then ground to a white powder in a small mill.In the worked examples, where reference is made to Silicate is meant acomposition substantially of the formula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}$

where a is 5.50, b is 0.3, c is 0, d is 0.7, n is 1 and M is Na. TABLE 1% w/w Silicate % w/w Na₂SO₄ % w/w Na₂CO₃ % w/w H₂O 81.54 18.46 0 0 80.2716.77 2.97 0 79.21 15.36 5.43 0 68.04 31.96 0 0 66.63 28.35 5.02 0 65.5125.48 9.01 0 79.91 18.09 0 2 78.66 16.43 2.91 2 77.62 15.05 5.33 2 66.6831.32 0 2 65.30 27.79 4.92 2 64.20 24.97 8.83 2 40.77 9.23 0 50  40.138.38 1.48 50  39.60 7.68 2.72 50  34.02 15.98 0 50  33.32 14.18 2.51 50 32.75 12.74 4.51 50  81.87 18.13 0 0 80.57 16.49 2.94 0 79.50 15.12 5.390 68.38 31.62 0 0 66.93 28.06 5.00 0 65.78 25.22 8.99 0 80.23 17.77 0 278.96 16.16 2.88 2 77.91 14.81 5.28 2 67.01 30.99 0 2 65.60 27.50 4.90 264.47 24.72 8.81 2 40.93 9.07 0 50  40.29 8.24 1.47 50  39.75 7.56 2.6950  34.19 15.81 0 50  33.47 14.03 2.50 50  32.89 12.61 4.50 50  82.0317.97 0 0 80.72 16.35 2.93 0 79.64 15.00 5.37 0 68.54 31.46 0 0 67.0827.92 5.00 0 65.92 25.10 8.98 0 80.39 17.61 0 2 79.11 16.02 2.87 2 78.0414.70 5.26 2 67.17 30.83 0 2 65.74 27.36 4.90 2 64.60 24.60 8.80 2 41.018.99 0 50  40.36 8.18 1.46 50  39.82 7.50 2.68 50  34.27 15.73 0 50 33.54 13.96 2.50 50  32.96 12.55 4.49 50  82.12 17.88 0 0 80.81* 16.27*2.92*  0* 79.72 14.92 5.35 0 68.64 31.36 0 0 67.17 27.83 4.99 0 66.0025.02 8.98 0 80.48 17.52 0 2 79.20** 15.94** 2.86**  2** 78.13 14.635.25 2 67.27 30.73 0 2 65.83 27.28 4.89 2 64.68 24.52 8.80 2 41.06 8.940 50  40.41 8.13 1.46 50  39.86 7.46 2.68 50  34.32 15.68 0 50  33.5913.92 2.50 50  33.00 12.51 4.49 50  82.18 17.82 0 0 80.87 16.22 2.91 079.78 14.88 5.35 0 68.71 31.29 0 0 67.23 27.78 4.99 0 66.05 24.97 8.97 080.54 17.46 0 2 79.25 15.89 2.86 2 78.18 14.58 5.24 2 67.33 30.67 0 265.89 27.22 4.89 2 64.73 24.47 8.79 2 41.09 8.91 0 50  40.44 8.11 1.4650  39.89 7.44 2.67 50  34.35 15.65 0 50  33.62 13.89 2.50 50  33.0312.49 4.49 50  82.33 17.67 0 0 81.01 16.08 2.90 0 79.91 14.76 5.32 068.87 31.13 0 0 67.38 27.64 4.98 0 66.19 24.85 8.96 0 80.69 17.31 0 279.39 15.76 2.84 2 78.31 14.47 5.22 2 67.49 30.51 0 2 66.03 27.08 4.88 264.86 24.35 8.78 2 41.17 8.83 0 50  40.51 8.04 1.45 50  39.96 7.38 2.6650  34.44 15.56 0 50  33.69 13.82 2.49 50  33.09 12.43 4.48 50  69.3830.62 0 0 67.83 27.20 4.97 0 66.60 24.47 8.93 0 81.15 16.85 0 2 79.8315.36 2.80 2 78.73 14.12 5.15 2 67.99 30.01 0 2 66.48 26.66 4.87 2 65.2723.98 8.75 2 41.40 8.60 0 50  40.73 7.84 1.43 50  40.17 7.20 2.63 50 34.69 15.31 0 50  33.92 13.60 2.48 50  33.30 12.23 4.47 50  82.80 17.200 0 81.46 15.68 2.86 0 80.34 14.40 5.26 0 70.12 29.88 0 0 68.50 26.564.94 0 67.20 23.91 8.89 0 81.80 16.20 0 2 80.45 14.80 2.75 2 79.32 13.625.06 2 68.71 29.29 0 2 67.13 26.03 4.84 2 65.86 23.43 8.71 2 41.73 8.270 50  41.05 7.55 1.40 50  40.47 6.95 2.58 50  35.06 14.94 0 50  34.2513.28 2.47 50  33.60 11.95 4.44 50  83.47 16.53 0 0 82.09 15.10 2.81 080.94 13.90 5.17 0

Example 2 Paraffin Wax Dispersions

Comparative 1

A paraffin wax dispersion was prepared by mixing 100 g of Laponite® RDsynthetic hectorite available from Rockwood Absorbents Limited, Widnes,England, in 5 l of hot paraffin wax. The dispersion was allowed to cooland then a sample was put into a closed jar, which was then closed andthen placed on the laboratory shelf. The jar was not in direct sunlight.After two months, the jar was revisited and it was noted that the samplehad turned a pale yellow colour, thereby indicating the discoloration ofthe dispersion.

Comparative 2

A paraffin wax dispersion was prepared by mixing 80 g of Laponite® RDsynthetic hectorite available from Rockwood Absorbents Limited, Widnes,England, 18 g of Na₂SO₄ and 2 g of Na₂CO₃ (particle size <10 microns,pre-mixed in microniser) in 5 l of hot paraffin wax. The dispersion wasallowed to cool and then a sample was put into a closed jar, which wasthen closed and then placed on the laboratory shelf. The jar was not indirect sunlight. After two months, the jar was revisited and it wasnoted that the sample had turned a pale yellow colour, therebyindicating the discoloration of the dispersion.

Synthetic Magnesium Silicate Invention

A paraffin wax dispersion was prepared by mixing 100 g of composition **(as indicated in Table 1) in 5 l of hot paraffin wax. The dispersion wasallowed to cool and then a sample was put into a closed jar, which wasthen closed and then placed on the laboratory shelf. The jar was not indirect sunlight. After two months, the jar was revisited and it wasnoted that the sample the dispersion was the same colour as it was twomonths before, thereby indicating no discoloration of the dispersion.

Example 3 Retention Aid

To demonstrate that the compositions of the present invention retainrheological properties similar to the prior art synthetic silicatecompositions, the performances as retention aids of the two compositionsof the invention * and ** were compared against Laponite RD (a synthetichectorite available from Rockwood Additives Limited, Widnes, England).The compositions were tested under total first pass retentionconditions, a standard procedure well known in the papermaking industry.When used in combination with a commercially available high molecularweight cationic polymeric retention aid, the results in Table 2 indicatethat the compositions of the present invention perform as effectively asLaponite RD as a retention aid. TABLE 2 Sample Total First PassRetention % Blank-no retention aid 71.8 Polymer 0.22 Kg/1000 Kg of paper75.7 produced Polymer 0.22 Kg/1000 Kg of paper 80 produced + Laponite RD0.44 Kg/1000 Kg of paper produced Polymer 0.22 Kg/1000 Kg of paper 80produced + Silicate* 0.44 Kg/1000 Kg of paper produced Polymer 0.22Kg/1000 Kg of paper 80 produced + Silicate* 0.44 Kg/1000 Kg of paperproduced

1. A synthetic magnesium silicate composition of the formula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}_{w}\{ {{Na}_{2}{SO}_{4}} \}_{x}\{ {{Na}_{2}{CO}_{3}} \}_{y}\{ {H_{2}O} \}_{z}$wherein a is 4.5 to <6.0, b is from >0 to 1.5, c is from 0 to 4,d=12−2a−b, M is a cation, n is the valency of the cation M, w is from32.8 to 94.7 wt. %, x is from 0.3 to 32.0 wt. %, y is from 0 to 9.0 wt.%, and z is from 0 to 50 wt. %.
 2. A process for the preparation of asynthetic magnesium silicate composition of the formula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}_{w}\{ {{Na}_{2}{SO}_{4}} \}_{x}\{ {{Na}_{2}{CO}_{3}} \}_{y}\{ {H_{2}O} \}_{z}$wherein a is 4.5 to <6.0, b is from >0 to 1.5, c is from 0 to 4,d=12−2a−b, M is Na or Li, n is 1, w is from 32.8 to 94.7 wt. %, x isfrom 0.3 to 32.0 wt. %, y is from 0 to 9.0 wt. %, and z is from 0 to 50wt. %, the process comprising: (i) forming an aqueous suspension ofmagnesium carbonate; (ii) forming a silica precipitate in the aqueoussuspension of magnesium carbonates; the proportions of magnesiumprovided by the magnesium carbonate and of silica precipitated in thesuspension corresponding to that of the formula of the magnesiumsilicate; (iii) whilst maintaining the resulting mixture of magnesiumcarbonate and silica in the wet state, subjecting it to hydrothermaltreatment by heating it in an aqueous medium and in the presence of theremaining constituents of the magnesium silicate in proportions withinthe ranges specified in the general formula thereof and in the presenceof excess dissolved sodium or lithium compound over that required toform the cation thereof until crystal growth occurs; (iv) separating thesolid and liquid phases; and (v) drying the resultant solid product at atemperature up to 450° C.
 3. A process for the preparation of asynthetic magnesium silicate composition of the formula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}_{w}\{ {{Na}_{2}{SO}_{4}} \}_{x}\{ {{Na}_{2}{CO}_{3}} \}_{y}\{ {H_{2}O} \}_{z}$wherein a is 4.5 to <6.0, b is from >0 to 1.5, c is from 0 to 4,d=12−2a−b, M is Na or Li, n is 1, w is from 32.8 to 94.7 wt. %, x isfrom 0.3 to 32.0 wt. %, y is from 0 to 9.0 wt. %, and z is from 0 to 50wt. %, the process comprising: (i) forming an aqueous slurry from (a) awater-soluble magnesium salt; (b) sodium silicate; (c) sodium carbonateor sodium hydroxide; and (d) material delivering lithium and fluorideions selected from the group consisting of (A) lithium fluoride and (B)a lithium compound in conjunction with hydrofluoric acid, fluosilicicacid, sodium silicofluoride or sodium fluoride, such that in the slurrythe following atomic ratios are present$\frac{Si}{F} = {0.5\quad{to}\quad 5.1}$$\frac{Li}{Mg} = {0.1\quad{to}\quad 1.0}$$\frac{Si}{{Mg} + {Li}} = {0.5\quad{to}\quad 1.5}$$\frac{Na}{{2\quad{Mg}} + {F\text{-}{Li}}} = {1.0\quad{to}\quad 2.0}$ the aqueous slurry being formed by co-precipitation by slowly combiningthe said magnesium salt and the said sodium silicate and the said sodiumcarbonate or sodium hydroxide, with heating and agitation, in an aqueousmedium which contains the said material or materials delivering thelithium and fluoride ions; (ii) taking the aqueous slurry so formed and,without washing free from soluble salts, hydrothermally treating it forabout 10 to 20 hours until crystal growth occurs; (iii) separating thesolid and liquid phases; and (iv) drying the resultant solid product ata temperature up to 450° C.
 4. A process for the preparation of asynthetic magnesium silicate composition of the formula:$\{ {\lbrack {{{Si}_{8}( {{Mg}_{a}{Li}_{b}} )}{O_{20}({OH})}_{4 - c}F_{c}} \rbrack^{d - \frac{d}{n}}M^{n +}} \}_{w}\{ {{Na}_{2}{SO}_{4}} \}_{x}\{ {{Na}_{2}{CO}_{3}} \}_{y}\{ {H_{2}O} \}_{z}$wherein a is 4.5 to <6.0, b is from >0 to 1.5, c is from 0 to 4,d=12−2a−b, M is Na or Li, n is 1, w is from 32.8 to 94.7 wt. %, x isfrom 0.3 to 32.0 wt. %, y is from 0 to 9.0 wt. %, and z is from 0 to 50wt. %, the process comprising: (i) precipitating a magnesium silicatehaving the value of “a” desired in the said composition by combining anaqueous solution of a water soluble magnesium salt with an aqueousalkaline solution of one or more sodium compounds in the presence ofdissolved silicon-delivering material, the pH of the alkaline solutionbeing maintained at 8 to 12.5 throughout; (ii) without first drying orwashing this precipitate heating it to a temperature of at least 170° C.and the pressure of at least 6.9 bar (100 psi), the temperature beingless than 370° C. and such that a liquid phrases present, until crystalgrowth occurs; (iii) separating the resultant solid and liquid phrases;and (iv) drying the resultant solid product.
 5. The composition of claim1, wherein a is from 5.30 to 5.68.
 6. The composition of claim 1,wherein z is less than
 2. 7. The composition of claim 1, wherein M isselected from Na, K, Li, an organic cation, and mixtures thereof.
 8. Thecomposition of claim 5, wherein z is less than
 2. 9. The composition ofclaim 5, wherein M is selected from Na, K, Li, an organic cation, andmixtures thereof.
 10. The composition of claim 6, wherein M is selectedfrom Na, K, Li, an organic cation, and mixtures thereof.
 11. The processof claim 2, wherein a is from 5.30 to 5.68.
 12. The process of claim 3,wherein a is from 5.30 to 5.68.
 13. The process of claim 4, wherein a isfrom 5.30 to 5.68.
 14. The process of claim 2, wherein z is less than 2.15. The process of claim 3, wherein z is less than
 2. 16. The process ofclaim 4, wherein z is less than
 2. 17. The process of claim 2, wherein Mis selected from Na, K, Li, an organic cation, and mixtures thereof. 18.The process of claim 3, wherein M is selected from Na, K, Li, an organiccation, and mixtures thereof.
 19. The process of claim 4, wherein M isselected from Na, K, Li, an organic cation, and mixtures thereof.